Rescue workers have recovered five bodies in southern Japan, bringing the death toll from heavy rains to seven, with five people missing feared dead and more than 20 others unaccounted for.

Troops and other rescuers reached some villages that had been cut off by torrential rain and rescued more than 300 stranded residents, officials said.

Heavy rain warnings are still in place for parts of the southern island of Kyushu after Typhoon Nanmadol swept across Japan earlier in the week, dumping large amounts of rain that damaged homes, roads and rice fields.

Japan's royal family postponed the formal announcement of Princess Mako's engagement to a college classmate on Saturday out of consideration for the suffering of people in the affected areas, palace officials said.

A new date has yet to be decided.

Chief cabinet secretary Yoshihide Suga said 12,000 troops and other rescuers were focusing on remote villages where hundreds are still stranded.

The operation has been slowed by mud and floodwaters, and more flooding is forecast in the country's east.

The body of an elderly woman was found washed up by a river that had overflowed in Oita, the prefectural disaster management department said.

Four other bodies were retrieved earlier on Friday in Asakura in Fukuoka prefecture, one of the hardest-hit areas.

Television footage showed rice fields and homes flooded after a river overflowed. Roads and bridges were damaged and dozens of vehicles and houses were destroyed.

Residents of remote villages were being airlifted by military helicopters while soldiers waded through floodwaters carrying elderly people on their backs.

Japan's Meteorological Agency said Fukuoka and Oita had experienced unprecedented amounts of rain.